Fuel cell systems of the initially named kind are in principle known, for example from the International Patent Application with the publication no. WO 99/05741.
With the development of alternative drive concepts for automotive applications the electric drive in combination with a low temperature fuel cell system and an electrochemical energy converter has attained particular importance. The choice of the fuel has an important influence on the complexity of the fuel cell system. When using organic fuels such as methanol or gasoline the complexity of the system increases because of the required conversion of the fuel to form a hydrogen rich gas. The use of pure hydrogen results in a substantial simplification of the system. The pure or contaminated hydrogen gas is then supplied to the fuel cell at the anode side in dependence on the load. At the cathode side oxygen or an oxygen containing gas, above all air, is supplied in accordance with the load. The desired value for the load results essentially from the behaviour of the total vehicle (accelerations, braking, etc.) desired by the driver and also from the power requirement of the electrical consumers which participate in the fuel cell system.
In a customary fuel cell system of the initially named kind air is supplied to the cathode side and consists, in addition to the desired oxygen component, of approximately 80% of nitrogen, which counts here as an inert gas and which forms the main component of the cathode exhaust gases (together with water vapour and a residual oxygen component. However, in the operation of the fuel cells, the nitrogen tends to diffuse through the membranes, which are present there, and appears as an undesired gas component at the anode side. Water also diffuses to the anode side so that the gases present there not only consist of the desired hydrogen component but rather also of the undesired nitrogen component and of water vapour. Accordingly it has previously been customary to dispose of at least a part of the anode exhaust gases at the anode side. Since these anode exhaust gases contain a residual component of hydrogen this must take place by a special catalytic combustion, since it would not be permissible to discharge hydrogen directly into the environment. As hydrogen is a valuable fuel attempts have also already been made to return a part of the anode exhaust gases to the anode side. Nevertheless, as a result of the nitrogen component of the anode exhaust gases which arises in the course of time, the anode exhaust gases must be disposed of by catalytic combustion at the anode side and replaced with fresh hydrogen.
In order to minimize the fuel consumption of a fuel cell vehicle the efficiency of a low temperature fuel cell has an important role to play. The degree of efficiency of a low temperature fuel cell depends, amongst other things, on the oxygen component of the supplied cathode gas and also on the proportion of oxygen in the supplied cathode gas and also in the proportion of hydrogen in the supplied anode gas, i.e. the system architecture which supplies the low temperature fuel cells with gas should be conceived so that the highest possible proportions of oxygen and hydrogen are present in the supplied gas flows. In the operation of low temperature fuel cells exhaust gases arise at the anode outlet which as a rule contained hydrogen. This hydrogen should ideally not leave the vehicle or the fuel cell system in order to achieve the requirement for “zero emissions”. Emission means in this context every element which is not present in the air.